Superconductivity in the Silicon Clathrate Compound (Na,Ba)xSi46

A new silicon clathrate compound containing barium, (Na,Ba) ${}_x$S${\mathrm{i}}_{46}$, becomes a type-II superconductor with a critical temperature $\left(T_{\mathrm{c}}\right)$ of about 4 K. In the compound, the silicon atoms are bonded tetrahedrally with about the same bond distances as in ordinary cubic diamond Si, but form 12- and 14-hedral cages which are linked by shared faces. The metal atoms are located in the center of the polyhedra. This is the first superconductor consisting of a Si ${\mathrm{sp}}^3$ covalent network.